1. The Seattle area is Zone 8

2. Local cultivated soils may often be deficient in nitrogen but are not likely to need supplementation with other minerals. Are you working on the basis of a soil test conducted by a soils lab? It is quite possible to overload your soil with nutrients and produce a toxicity; phosphorus effectively does not leach so that over-applications of that commonly used primary nutrient have to be corrected by removal and replacement of the soil so affected. Kitchen gardening references that babble about pouring on compost and nutrients without limit are wrong, even the mineral content of composted plant debris, kitchen scraps etc. can soon produce an excess.

I want to mix my own fertilizer for growing seedlings.

Seattle area is zone 8, but local area where I live by conditions is much more zone 7.

I always used Espoma brand which is what was available here in regular gardening centers & box stores.
You can get it online but shipping is usually cost prohibitive.
Both greensand and rock phosphate are listed in Product section under Organic Supplements.
Don't know where you are in reference to Seattle but you only need enter your zip code to find places where it may be available.
I used the greensand as one of MANY things to try loosening up hard clay here in New Jersey. Greensand is naturally found in Southern Jersey while I live in the clay capitol of the state.
Good Luck.

Here is a link that might be useful: Zip 98101 places where Espoma products available

If you are in Brier you are well within USDA 8. Many people do not understand the USDA Hardiness Zones, which are based on average annual minimum temperatures for specific 15 year periods. To be in USDA 7 you have to have had a 15 year average that falls between 0F and 10F, not occasional temperatures within this range.

If you compare zone maps of western WA USDA 8 pretty much lines up with Sunset 4 and 5, with Sunset 4 being USDA 8a and Sunset 5 being USDA 8b. With both the USDA Hardiness Zones and the Sunset Climate Zones the designators are using official climate records to form their interpretations. Where the USDA mapping sometimes falls apart noticeably is here in the mountainous west where the zones are drawn stretched into contiguous bands, resulting in lowlands and mountains being shown in the same zones part of the time.

This post was edited by bboy on Wed, Jan 15, 14 at 16:08

not sure if it is a "best" source, but Sky nursery lists that they carry those.
Way long ago, PCC used to have those, and then there was another guy who sold those up on 99 in south everett. but I've long since forgotten the name.
Maybe even Manna milling in Mountlake terrace may know a source.
And coop supply in Everett may carry them.
And what about all the new "Green" growers and their supplies-- saw one somewhere along 99 in Lynnwood that has grow lights etc.

Not sure where you are... but if you have a Grange or feed supply store near you, check with them. I used to get that kind of stuff at the Grange in Redmond before they closed down. Issaquah also has that kind of stuff, but a little more expensive. Often these grange and feed stores are cheaper than the nurseries that sell the same stuff in smaller bags.

But I would also question whether or not you even need it. A good seed mix doesn't need or want much for fertilizers, its more about the soil texture and drainage/water retention than it is about fertility initially. The seeds themselves provide most of what they need, and if you DO need a fertilizer, my experience is you are better with a liquid low level fert than the typical stuff you would make from these ingredients. Fish tank water worked the best for me actually. Fish emulsion or seaweed tea is supposed to work pretty well too. These will get used quickly, and flush out before building up too much.

Just my two cents. I rarely do seeds though, so maybe not worth so much.

I've purchased at Skagit Farmer's Coop (now Country Store) when we were nearby. At both the Stanwood & Freeland stores I've seen a wide range of soil amendments there at reasonable prices as compared to feed stores in the SE King county area.

In Seattle, Walt's Organics has premixed as well as individual ingredients: http://www.waltsorganic.com/index.html

Here is a link that might be useful: Country Store

The major source of rock phosphate (phosphorous source) is from strip mining in the heart of Florida's natural and agricultural lands. Nearly half a million acres have been destroyed and the devastation continues as we speak. There are other sources for this component of fertilizer. E.g. in milorganite.

With a lot of it being sold to people that don't need it.